This invention relates to dynamoelectric machines and more particularly to drum type homopolar machines.
Drum type homopolar dynamoelectric machines include a stationary excitation system and a rotating drum composed of a combination of ferromagnetic and highly conductive materials configured such that a direct current output voltage is produced along the axial length of the drum. These machines incorporate a set of current collection members at either axial end of the rotor, which carries full load current. Homopolar dynamoelectic machines may operate as either a motor or generator and are particularly suited to transfer energy in short, high current pulses to a storage inductor and a final load consisting of a resistive-inductive system, as for example, an electromagnetic projectile launcher. The rotor of drum type homopolar machines may include a cylindrical shell of a highly conductive, non-ferromagnetic material which generates and supports the full load current. This member is bonded or shrunk onto a ferromagnetic inner cylindrical core which serves as the main rotor body and is directly attached to a drive or input shaft. Both components of the rotor are, preferably, homogeneous materials without segmentation or any combination of axial or circumferential grooves. Since modern current collectors may operate at a current density of between 10 and 15 kiloamps per square inch, it is imperative that the rotor surface on the two axial ends be smooth since this zone is used exclusively for current collection with, for example, solid metal-graphite brushes. The machine's internal electromotive force is confined to an axial zone along the center of the rotor between the two outer current collection zones.
Drum type homopolar machines may be classified as truncated drum or full drum types according to the relative rotor active length. The excitation system includes a stator having a main pole which is used to confine the magnetic flux to a zone of the rotor which is directly in line radially with the main pole piece. It is desirable that the total machine flux should only cut the rotor surface at a location which is separated from the current collecting zone. In practical machines, with significant iron-iron air gaps, magnetic saturation of the core material or poles and conventional pole tip geometries, an amount of leakage flux will typically pass from the main pole said across the air gap at a non-radial angle and enter the rotor magnetic circuit through the current collection zone. It is this leakage flux which causes an additional voltage to be generated in the rotor zone under the brushes. The particular construction of a rotor shell which includes a continuous homogeneous cylinder in conjunction with the use of a relatively long brush collector at each end creates additional induced electromotive force due to the leakage flux that results in large continuous circulating current in closed, short circulating loops composed of the rotor conductor and each brush box at every point along the circumference.
In the design of both truncated drum and full drum, conventional homopolar machinery, corrective measures have been implemented to reduce the severity of the stray magnetic field not directly forming in the active air gap area. Certain techniques for improving machine reliability although not performance, reduce the thermal stress associated with higher than average brush-to-rotor circulating currents and the consequent, sometimes unpredictable, temperature rises. Some of the conventional techniques used previously for reducing stray air gap magnetic fields are as follows:
1. Keeping the collector length as small as possible by increasing the collector current density;
2. Increasing the rotor diameter of the machine with a significant decrease in rotor collector length;
3. Changing the radial thickness of the conductor rotor shell between active and collector zones so that the active zone contains the rotor shell segment with the least amount of non-ferromagnetic material;
4. Using a solid ferromagnetic rotor without a conductive shell but plating the rotor core in the current collection zone with a highly conductive material so as to maintain a minimum air gap in the active region;
5. Shielding the current collection system with a non-ferromagnetic, highly electrically conductive enclosure and necessitating that surface eddy currents provide screening for those applications which are strictly of a time transient or pulsed operation; and
6. Extending the null-flux zone of the current collection area, but adding a necessary axial length to the stator frame which usually results in large unutilized air spaces in the machine.
Other methods which do not generally decrease the stray magnetic field but tend to lessen the possibility of heavy circulating currents include:
1. Attaching each current collection module to separate load circuits or extending the lead length of individual brush modules so as to increase the effective resistance of this circulating current path; and
2. Modifying the rotor to increase the effective rotor surface axial resistance path for circulating currents above the resistance encountered for currents flowing in a singularly radial direction.
In the interest of building lightweight and extremely compact machine designs, often with a low moment of inertia, all of the above conventional methods have proven to be cumbersone and unable to meet minimum weight criteria. In assessing any conventional homopolar generator, a significant percentage of total field magnetomotive force or ampere turns directly contributes to magnetizing the rotor in undesirable zones and even in locations such as the bearing supports. A copending commonly assigned application Ser. No. 677,768, filed Dec. 3, 1984, and entitled "HOMOPOLAR DYNAMOELECTRIC MACHINE WITH A SHIELDED STATOR EXCITATION COIL", discloses a homopolar machine having a flux shield on the main stator excitation coil to reduce stray magnetic field in the current collection zone and is hereby incorporated by reference.